1. Field of the Invention
This invention relates generally to seals, and more particularly to a magnetic fluid seal assembly which is advantageously used to seal a small annular clearance between two relatively rotatable parts.
2. Description of the Prior Art
In small-sized high precision motors, it is often required to use such a seal or seals as would enable effective sealing between two relatively rotatable parts without hindering smooth rotation therebetween. One typical example of seals meeting such a requirement is the so-called "magnetic fluid seal" which utilizes ferromagnetic particles dispersed in a carrier fluid such as oil.
A magnetic fluid seal is disclosed for example in U.S. Pat. No. 4,407,508 to Raj et al. According this U.S. patent, the seal comprises an annular pole piece surrounding a rotary shaft with a small annular clearance formed therebetween. The pole piece is held in contact with permanent magnet, so that a magnet flux runs within the annular clearance to magnetically retain a ferromagnetic fluid loaded into the clearance.
Because of its fluidity, the ferromagnetic fluid ensures smooth rotation of the shaft while effectively sealing the annular clearance. However, such fluidity of the ferromagnetic fluid also gives rise to a possibility of being centrifugally scattered when rotationally moved with the shaft, causing a loss of the expensive material and more importantly resulting in contamination of the surrounding.
Particularly in spindle motors used for driving data carrier discs (magnetic discs, optical discs, etc.), the contamination of the space adjacent the disc should be avoided because this may hinder reliable write-in and read-out operation. Therefore, some countermeasure to prevent scattering of ferromagnetic fluid is absolutely necessary when it is used to provide a seal for the spindle motor.